i think that kogmaw should be play like master yi , so i made a build similar to him.With Damage per second and lifesteal,it definitely ensure you of getting kills.

For Rune,just stick with normal ad runes ,armor and magic resist . This will let you survive in 1on1 against mage or tanky hero such as darius.

Buying Malady will be able to get Attack speed which is 50% and also lower magic resist from its passive (for ur ultimate).So i think that malady is a good buy.Next, buying blood thirster will increase ur damage output and destroy hero in teamfight.

I think that have exhaust will help you to keep in with your enermy ,while flash is a good escape tool as kogmaw usually dont have escape tool except his 3rd skill.

At level 1 : add your 1st skill. After that,max ur 2nd skill immediately for more range and magic damage.Next add 1st to have more attack speed passive.lastly max your 3rd skill and add your ultimate at given levels.

Pros: Farm alot,able to carry team very well,long range,able to comeback even if you feed at early
Cons:no escape tool.
However overall it is awesome no matter what.